Answer
From top to bottom:
$80$ $mg$
$40$ $mg$
$20$ $mg$
$10$ $mg$
$5$ $mg$
Work Step by Step
After one half-life, the mass is halved.
Initial mass: 80 mg.
1 half-life: $80$ $mg \times \frac{1}{2} = 40$ $mg$
2 half-lives: $40$ $mg \times \frac{1}{2} = 20$ $mg$
3 half-lives: $20$ $mg \times \frac{1}{2} = 10$ $mg$
4 half-lives: $10$ $mg \times \frac{1}{2} = 5$ $mg$
Therefore, all the masses of iodine-131 on the vertical axis are (from top to bottom):
$80$ $mg$
$40$ $mg$
$20$ $mg$
$10$ $mg$
$5$ $mg$
